1. Field of the Invention
The present invention relates to an optically scanning apparatus for optically scanning the surface of a sample by m×n (m and n being natural numbers of 2 or more) number of sub beams and a defect inspection system for detecting a defect existing on the surface of a sample using this optically scanning apparatus.
2. Description of the Related Art
Along with the increasing miniaturization of LSIs (large scale integrations), there has been strong demand for development of a defect inspection system able to accurately detect micro defects existing on the surface of a semiconductor wafer, patterned wafer, mask blank, etc. In particular, there has been strong demand for development of a defect inspection system able to accurately detect micro defects since the manufacturing yield falls by a large margin when defects are present on a semiconductor wafer before processing for fabrication of semiconductor devices.
In the past, as a system for inspecting a semiconductor wafer for defects, there has been a defect inspection system using laser scattered light. In a defect inspection system using laser scattered light, the sample to be inspected for defects is placed on a rotating sample stage, a single laser beam is projected toward the sample, and the light scattered from the surface of the sample is detected to inspect for defects.
As another defect inspection system, there is known a defect inspection system disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-199031 previously proposed by the present assignee. In this known defect inspection system, a light beam generated from a laser light source is converted by a one-dimensional diffraction grating to n number of sub beams arranged in lines. The surface of the sample is raster scanned by these n number of sub beams. The light of the n number of sub beams reflected from the surface of the sample are received by a photosensor having n number of light receiving elements. The output signals of the light receiving elements are compared with each other to detect defects. A defect inspection system utilizing such sub beams uses a confocal optical system, so achieves a high resolution and can accurately defect micro defects of for example about 50 nm.
Summarizing the problem to be solved by the invention, a defect inspection system of the laser scattered light type places the sample on a rotating sample stage and projects a single laser beam toward the rotating sample, so the entire surface of the sample can be scanned in a relatively short time. Therefore, there is the effect that the inspection time can be shortened. The surface of a semiconductor wafer, however, has micro step differences or unevenness of an atomic level, so the intensity of the light scattered from the surface of a normal sample is too strong and scattered light due to defects present on the surface of the sample is difficult to differentiated from the light scattered from a normal surface. As a result, the resolution of detection of defects becomes low. In particular, in the case of a wafer annealed by hydrogen or a semiconductor wafer formed with an epitaxial layer on the surface, the surface of the wafer has surface roughness on an atomic level, so scattered light is produced not due to defects. To reduce the effects of this scattered light, the detection sensitivity has to be lowered and detection of micro defects of about 50 nm size becomes difficult.
On the other hand, a defect inspection system of the multi beam system uses a confocal optical system, so achieves a high resolution and can accurately detect defects of even about 50 nm size. Since it raster scans the surface of a sample by a plurality of light beams arranged in lines, however, a relatively long time is required to scan the entire surface of a sample and the inspection time becomes long.